The present invention relates to waste treatment facilities which burn municipal waste. Typical waste-to-energy ("WTE") plants burn municipal solid waste for the purpose of volume reduction of the waste prior to landfill disposal. The heat generated in the combustion process is typically recovered in a special design heat recovery boiler where it produces steam. The steam is then either condensed directly for reintroduction into the cycle as boiler feedwater, or is economically exploited in the form of process steam or used to produce electrical power via a steam turbine generator set. The present invention particularly relates to WTE plants which utilize the heat of combustion of the waste to generate electrical power.
In order to obtain a reasonable power generation efficiency, the steam is required to be superheated in a superheater which is typically an integral part of the heat recovery boiler. Typical municipal solid waste ("MSW") contains many components whose combustion results in the formation of acid gases. Acid gases are highly corrosive to the metal components of the heat recovery boiler, with corrosion effects increasing with metal temperature. Boiler component metal temperatures are basically a function of the inside water or steam temperature rather than the outside flue gas temperature. Therefore, in order to reduce the effect of acid gas corrosion attack on the boiler components, specifically the superheater tubes, WTE plant heat recovery boilers operate at rather low steam conditions. Typical steam pressures range from 600 to 1000 psig, with associated saturation temperatures of about 500.degree. to 650.degree. F. Typical main steam temperatures in WTE plants are kept below 800.degree. F. resulting in superheater metal temperatures below 1000.degree. F. It is known that a single cycle WTE facility operating at relatively low steam conditions will achieve a relatively low overall thermal cycle efficiency, in the neighborhood of 25%. Despite these measures, superheater corrosion in WTE plant heat recovery boilers is a common problem, requiring frequent superheater repairs and replacements.
It is generally known that combined cycle power plants, combining a steam turbine-generator set with a combustion turbine-generator set can achieve thermal cycle efficiencies up to almost twice as high as low pressure, low temperature single cycle power plants.
U.S. Pat. No. 4,882,903 to Lowry Jr. et al. discloses a combined cycle waste-to-energy plant including a steam turbine-generator set and a combustion turbine-generator set. Lowry states that the low efficiency of conventional waste-to-energy facilities is attributable to the fact that much of the heat derived from combustion of the waste is expended in heating the combustion air delivered to the incinerator and in drying and heating moisture contained in the waste (col. 1, lines 20-25). Lowry discloses utilizing the hot exhaust gas from the combustion turbine to dry and heat the moisture containing waste and the combustion air. Upon exiting the combustion turbine, the hot exhaust gas is passed through a steam superheater to reduce its temperature from approximately 1000.degree. F. to 600.degree. F., then the 600.degree. F. exhaust gas flows to the incinerator. A portion of the 600.degree. F. exhaust gas may also be fed into the waste heat recovery boiler (col. 2, lines 42-50). Lowry also states that the products of combustion of some wastes are corrosive to alloy steels used to achieve high steam superheat temperatures (col. 1, lines 31-34), and that the disclosed plant utilizes only the high temperature exhaust gases discharged from the combustion turbine to produce superheated steam (col. 7, lines 25-29). While Lowry alludes to an increase in efficiency resulting from the use of combustion turbine exhaust gas to dry and heat the waste and incinerator inlet air, no actual examples of thermal efficiencies are given. In fact, Lowry gives no working examples at all, and does not address working pressures or percentage efficiency attained.
U.S. Pat. No. 4,957,049 to Strohmeyer, Jr. discloses a combined cycle steam plant integrated with a sludge incineration process wherein the heat generated in the sludge combustor reheats the gas turbine exhaust gas at a mid-point of the overall cycle (col. 2, lines 25-30 and col. 3, lines 3-6). Immediately upon exiting the gas turbine, the gas turbine exhaust is fed to a heat recovery boiler which includes boiler and superheater surfaces (col. 3, lines 19-22). The heat recovery boiler feeds a steam turbine-generator set. Upon exiting the heat recovery boiler, a portion of the gas turbine exhaust may be fed to a sludge pellet dryer while a parallel portion of the gas turbine exhaust is used to support the combustion of the dried sludge pellets. The heat input from the sludge pellet firing system regenerates the gas turbine exhaust stream, which is then fed into a second heat recovery boiler, which includes economizer, boiler and superheater surfaces. The second heat recovery boiler also feeds steam to the steam turbine-generator set. In the sole example given, Strohmeyer discloses an operating pressure of 650 psig in the first waste heat boiler and 360 psig in the second waste heat boiler. Higher operating pressures are not addressed or suggested.
U.S. Pat. No. 4,852,344 to Warner discloses a combined cycle waste-to-energy plant wherein the gas turbine exhaust may be fed to the waste incinerator or may be combined with the incinerator outlet gas stream. The gas turbine exhaust may also be fed directly to a heat recovery boiler, which includes boiler and superheater surfaces. In the only operating example discussed, Warner discloses a boiler steam conditions of 685 psig/755.degree. F. and a steam turbine conditions of 650 psig/750.degree. F. In the background section of the specification, Warner states that an efficiency increase in the order of 8% could be realized at steam conditions of about 1000 psig and 840.degree. F., but excessive high temperature corrosion of boiler surfaces will result due to acids formed by the waste incineration (col. 1, lines 16-22).
U.S. Pat. No. 5,072,675 to Fowler discloses a process for waste treatment which includes 1) passing organic waste into a chamber; 2) pumping an inert gas into the chamber until the pressure within the chamber is at least 10,000 psi; 3) heating the chamber to over 300.degree. F.; 4) dissociating the organic waste into gaseous constituents; 5) passing the gaseous constituents through a turbine to produce electricity; 6) then passing the gaseous constituents into what is referred to as a boiler; 7) mixing oxygen containing gas, such as air, with the gaseous constituents within the boiler so as to produce what is referred to as steam; 8) passing the steam through a turbine to produce electricity. While the Fowler patent does disclose a waste treatment facility having two turbines, it does not address the problem of waste heat recovery boiler superheater surface corrosion or overall thermal cycle efficiency.
There has been a long felt need in the WTE field for a system and method of operation which effect increased superheater temperatures, increased pressures and increased thermal cycle efficiencies while minimizing the aforementioned corrosion problems inherent in all WTE facilities.